In order to keep the air/fuel ratio of an internal combustion engine constant, in particular during non-steady-state operation, the quantity of fuel to be injected which corresponds to the air charge of a cylinder must be corrected by a value which takes into account the build up or reduction of the fuel wall film deposited in the inlet pipe and on the inlet valves. German Published Patent Application No. 39 39 548 A describes such a wall film compensation scheme.
The selection of the parameters of such a method is, however, dependent on the state of aging of the engine and on the type of fuel used. The wall film behavior during non-steady-state operation can change significantly as a result of inlet valve carbonization or as a result of filling the fuel tank with a different fuel from that used to calibrate the method. As a result, exhaust gas emissions and driving behavior are worsened.
German Published Patent Applications Nos. 42 43 449 A and 43 23 244 A describe adaptation methods which, on the basis of the variation in the air/fuel ratio during non-steady-state operation, adapt the wall film compensation parameters during operation to the respective fuel and to the carbonization state of the engine. However, these methods require the use of a linear lambda sensor which is significantly more expensive than the Nernst probes which are customary at the present time.
In German Published Patent Application No. 41 15 211 A, a different method is illustrated which avoids the aforementioned disadvantage. However, in accordance with that reference, there is a requirement for the lambda control to be switched off during a non-steady-state process, which can lead to worsening of the exhaust gas emissions.